NAME

SYNOPSIS

DESCRIPTION

ptrace() provides tracing and debugging facilities. It allows one process
(the tracing process) to control another (the traced process). Most of
the time, the traced process runs normally, but when it receives a signal
(see sigaction(2)), it stops. The tracing process is expected to notice
this via wait(2) or the delivery of a SIGCHLD signal, examine the state
of the stopped process, and cause it to terminate or continue as ap-
propriate. ptrace() is the mechanism by which all this happens. ptrace()
is only available on kernels compiled with the PTRACE option.
The request argument specifies what operation is being performed; the
meaning of the rest of the arguments depends on the operation, but except
for one special case noted below, all ptrace() calls are made by the
tracing process, and the pid argument specifies the process ID of the
traced process. request can be:
PT_TRACE_ME
This request is the only one used by the traced process; it declares
that the process expects to be traced by its parent. All the other
arguments are ignored. (If the parent process does not expect to
trace the child, it will probably be rather confused by the results;
once the traced process stops, it cannot be made to continue except
via ptrace().) When a process has used this request and calls
execve(2) or any of the routines built on it (such as execv(3)), it
will stop before executing the first instruction of the new image.
Also, any setuid or setgid bits on the executable being executed
will be ignored.
PT_READ_I, PT_READ_D
These requests read a single int of data from the traced process'
address space. Traditionally, ptrace() has allowed for machines with
distinct address spaces for instruction and data, which is why there
are two requests: conceptually, PT_READ_I reads from the instruction
space and PT_READ_D reads from the data space. In the current
OpenBSD implementation, these two requests are completely identical.
The addr argument specifies the address (in the traced process' vir-
tual address space) at which the read is to be done. This address
does not have to meet any alignment constraints. The value read is
returned as the return value from ptrace().
PT_WRITE_I, PT_WRITE_D
These requests parallel PT_READ_I and PT_READ_D, except that they
write rather than read. The data argument supplies the value to be
written.
PT_CONTINUE
The traced process continues execution. addr is an address specify-
ing the place where execution is to be resumed (a new value for the
program counter), or (caddr_t)1 to indicate that execution is to
pick up where it left off. data provides a signal number to be
delivered to the traced process as it resumes execution, or 0 if no
signal is to be sent.
PT_KILL
The traced process terminates, as if PT_CONTINUE had been used with
SIGKILL given as the signal to be delivered.
PT_ATTACH
This request allows a process to gain control of an otherwise unre-
lated process and begin tracing it. It does not need any cooperation
from the to-be-traced process. In this case, pid specifies the pro-
cess ID of the to-be-traced process, and the other two arguments are
ignored. This request requires that the target process must have the
same real UID as the tracing process, and that it must not be exe-
cuting a set-user-ID or set-group-ID executable. (If the tracing
process is running as root, these restrictions do not apply.) The
tracing process will see the newly traced process stop and may then
control it as if it had been traced all along.
PT_DETACH
This request is like PT_CONTINUE, except that it does not allow
specifying an alternate place to continue execution, and after it
succeeds, the traced process is no longer traced and continues exe-
cution normally.
PT_IO
This request is a more general interface that can be used instead of
PT_READ_D, PT_WRITE_D, PT_READ_I and PT_WRITE_I. The I/O request is
encoded in a "struct ptrace_io_desc" defined as:
struct ptrace_io_desc {
int piod_op;
void *piod_offs;
void *piod_addr;
size_t piod_len;
};
Where piod_offs is the offset within the traced process where the
I/O operation should be made, piod_addr is the buffer in the parent
and piod_len is the length of the I/O request. The piod_op member
specifies what operation needs to be done. Possible values are:
PIOD_READ_D
PIOD_WRITE_D
PIOD_READ_I
PIOD_WRITE_I
See also the description of PT_READ_I for the difference between D
and I spaces. A pointer to the descriptor is passed in addr. On re-
turn the piod_len field in the descriptor will be updated with the
actual number of bytes transferred. If the requested I/O couldn't be
successfully performed ptrace() will return -1 and set errno.
PT_SET_EVENT_MASK
This request can be used to specify which events in the traced pro-
cess should be reported to the tracing process. These events are
specified in a "struct ptrace_event" defined as:
typedef struct ptrace_event {
int pe_set_event;
} ptrace_event_t;
Where pe_set_event is the set of events to be reported. This set is
formed by OR'ing together the following values:
PTRACE_FORK
Report fork(2).
A pointer to this structure is passed in addr. The data argument
should be set to sizeof(struct ptrace_event).
PT_GET_EVENT_MASK
This request can be used to determine which events in the traced
process will be reported. The information is read into the "struct
ptrace_event" pointed to by addr. The data argument should be set to
sizeof(struct ptrace_event).
PT_GET_PROCESS_STATE
This request reads the state information associated with the event
that stopped the traced process. The information is reported in a
"struct ptrace_state" defined as:
typedef struct ptrace_state {
int pe_report_event;
pid_t pe_other_pid;
} ptrace_state_t;
Where pe_report_event is the event being reported. If the event be-
ing reported is PTRACE_FORK, pe_other_pid will be set to the process
ID of the other end of the fork. A pointer to this structure is
passed in addr. The data argument should be set to sizeof(struct
ptrace_state).
Additionally, machine-specific requests can exist. All OpenBSD platforms
support the following requests:
PT_GETREGS
This request reads the traced process' machine registers into the
"struct reg" (defined in <machine/reg.h>) pointed to by addr.
PT_SETREGS
This request is the converse of PT_GETREGS; it loads the traced pro-
cess' machine registers from the "struct reg" (defined in
<machine/reg.h>) pointed to by addr.
In addition, all platforms but luna88k, mvme88k, sgi and vax support
these additional requests:
PT_GETFPREGS
This request reads the traced process' floating-point registers into
the "struct fpreg" (defined in <machine/reg.h>) pointed to by addr.
PT_SETFPREGS
This request is the converse of PT_GETFPREGS; it loads the traced
process' floating-point registers from the "struct fpreg" (defined
in <machine/reg.h>) pointed to by addr.
The following requests are available on i386:
PT_GETXMMREGS
This request reads the traced process' XMM registers into the
"struct xmmregs" (defined in <machine/reg.h>) pointed to by addr.
PT_SETXMMREGS
This request is the converse of PT_GETXMMREGS; it loads the traced
process' XMM registers from the "struct xmmregs" (defined in
<machine/reg.h>) pointed to by addr.
Finally, the following request is available on sparc and sparc64:
PT_WCOOKIE
This request reads the traced process' 'window cookie' into the int
pointed to by addr. The window cookie needs to be 'XOR'ed' to
stack-saved program counters.

ERRORS

Some requests can cause ptrace() to return -1 as a non-error value; to
disambiguate, errno is set to zero and this should be checked. The possi-
ble errors are:
[ESRCH]
No process having the specified process ID exists.
[EINVAL]
• A process attempted to use PT_ATTACH on itself.
• The request was not one of the legal requests.
• The signal number (in data) to PT_CONTINUE was neither 0 nor a
legal signal number.
• PT_GETREGS, PT_SETREGS, PT_GETFPREGS, or PT_SETFPREGS was at-
tempted on a process with no valid register set. (This is nor-
mally true only of system processes.)
[EBUSY]
• PT_ATTACH was attempted on a process that was already being
traced.
• A request attempted to manipulate a process that was being
traced by some process other than the one making the request.
• A request (other than PT_ATTACH) specified a process that
wasn't stopped.
[EPERM]
• A request (other than PT_ATTACH) attempted to manipulate a pro-
cess that wasn't being traced at all.
• An attempt was made to use PT_ATTACH on a process in violation
of the requirements listed under PT_ATTACH above.
• An attempt was made to use PT_ATTACH on a system process.

BUGS

On the SPARC, the PC is set to the provided PC value for PT_CONTINUE and
similar calls, but the NPC is set willy-nilly to 4 greater than the PC
value. Using PT_GETREGS and PT_SETREGS to modify the PC, passing
(caddr_t)1 to ptrace(), should be able to sidestep this.
Single-stepping is not available.
MirOS BSD #10-current March 2, 2004 3